This PEP describes the "range literal" proposal for Python 2.0.
This PEP tracks the status and ownership of this feature, slated
for introduction in Python 2.0. It contains a description of the
feature and outlines changes necessary to support the feature.
This PEP summarizes discussions held in mailing list forums, and
provides URLs for further information, where appropriate. The CVS
revision history of this file contains the definitive historical
record.

However, this solution is not always prudent. Firstly, problems
arise when altering the sequence in the body of the for-loop,
resulting in the for-loop skipping items. Secondly, it is not
possible to iterate over, say, every second element of the
sequence. And thirdly, it is sometimes necessary to process an
element based on its index, which is not readily available in the
above construct.

For these instances, and others where a range of numbers is
desired, Python provides the range builtin function, which
creates a list of numbers. The range function takes three
arguments, start, end and step. start and step are
optional, and default to 0 and 1, respectively.

The range function creates a list of numbers, starting at
start, with a step of step, up to, but not including end, so
that range(10) produces a list that has exactly 10 items, the
numbers 0 through 9.

Using the range function, the above example would look like
this:

>>> for i in range(len(l)):
... print l[i]
a
b
c
d

Or, to start at the second element of l and processing only
every second element from then on:

>>> for i in range(1, len(l), 2):
... print l[i]
b
d

There are several disadvantages with this approach:

Clarity of purpose: Adding another function call, possibly with
extra arithmetic to determine the desired length and step of the
list, does not improve readability of the code. Also, it is
possible to "shadow" the builtin range function by supplying a
local or global variable with the same name, effectively
replacing it. This may or may not be a desired effect.

Efficiency: because the range function can be overridden, the
Python compiler cannot make assumptions about the for-loop, and
has to maintain a separate loop counter.

Consistency: There already is a syntax that is used to denote
ranges, as shown below. This syntax uses the exact same
arguments, though all optional, in the exact same way. It seems
logical to extend this syntax to ranges, to form "range
literals".

In Python, a sequence can be indexed in one of two ways:
retrieving a single item, or retrieving a range of items.
Retrieving a range of items results in a new object of the same
type as the original sequence, containing zero or more items from
the original sequence. This is done using a "range notation":

>>> l[2:4]
['c', 'd']

This range notation consists of zero, one or two indices separated
by a colon. The first index is the start index, the second the
end. When either is left out, they default to respectively the
start and the end of the sequence.

There is also an extended range notation, which incorporates
step as well. Though this notation is not currently supported
by most builtin types, if it were, it would work as follows:

>>> l[1:4:2]
['b', 'd']

The third "argument" to the slice syntax is exactly the same as
the step argument to range(). The underlying mechanisms of the
standard, and these extended slices, are sufficiently different
and inconsistent that many classes and extensions outside of
mathematical packages do not implement support for the extended
variant. While this should be resolved, it is beyond the scope of
this PEP.

Extended slices do show, however, that there is already a
perfectly valid and applicable syntax to denote ranges in a way
that solve all of the earlier stated disadvantages of the use of
the range() function:

It is clearer, more concise syntax, which has already proven to
be both intuitive and easy to learn.

It is consistent with the other use of ranges in Python
(e.g. slices).

Because it is built-in syntax, instead of a builtin function, it
cannot be overridden. This means both that a viewer can be
certain about what the code does, and that an optimizer will not
have to worry about range() being "shadowed".

There is one minor difference between range literals and the slice
syntax: though it is possible to omit all of start, end and
step in slices, it does not make sense to omit end in range
literals. In slices, end would default to the end of the list,
but this has no meaning in range literals.

The proposed implementation can be found on SourceForge [1]. It
adds a new bytecode, BUILD_RANGE, that takes three arguments from
the stack and builds a list on the bases of those. The list is
pushed back on the stack.

The use of a new bytecode is necessary to be able to build ranges
based on other calculations, whose outcome is not known at compile
time.

The code introduces two new functions to listobject.c, which are
currently hovering between private functions and full-fledged API
calls.

PyList_FromRange() builds a list from start, end and step,
returning NULL if an error occurs. Its prototype is:

PyObject * PyList_FromRange(long start, long end, long step)

PyList_GetLenOfRange() is a helper function used to determine the
length of a range. Previously, it was a static function in
bltinmodule.c, but is now necessary in both listobject.c and
bltinmodule.c (for xrange). It is made non-static solely to avoid
code duplication. Its prototype is:

One possible solution to the discrepancy of requiring the end
argument in range literals is to allow the range syntax to
create a "generator", rather than a list, such as the xrange
builtin function does. However, a generator would not be a
list, and it would be impossible, for instance, to assign to
items in the generator, or append to it.

The range syntax could conceivably be extended to include tuples
(i.e. immutable lists), which could then be safely implemented
as generators. This may be a desirable solution, especially for
large number arrays: generators require very little in the way
of storage and initialization, and there is only a small
performance impact in calculating and creating the appropriate
number on request. (TBD: is there any at all? Cursory testing
suggests equal performance even in the case of ranges of length
1)

However, even if idea was adopted, would it be wise to "special
case" the second argument, making it optional in one instance of
the syntax, and non-optional in other cases ?

Should it be possible to mix range syntax with normal list
literals, creating a single list? E.g.:

>>> [5, 6, 1:6, 7, 9]

to create:

[5, 6, 1, 2, 3, 4, 5, 7, 9]

How should range literals interact with another proposed new
feature, "list comprehensions" [2]? Specifically, should it be
possible to create lists in list comprehensions? E.g.:

>>> [x:y for x in (1, 2) y in (3, 4)]

Should this example return a single list with multiple ranges:

[1, 2, 1, 2, 3, 2, 2, 3]

Or a list of lists, like so:

[[1, 2], [1, 2, 3], [2]_, [2, 3]]

However, as the syntax and semantics of list comprehensions are
still subject of hot debate, these issues are probably best
addressed by the "list comprehensions" PEP.

Range literals accept objects other than integers: it performs
PyInt_AsLong() on the objects passed in, so as long as the
objects can be coerced into integers, they will be accepted.
The resulting list, however, is always composed of standard
integers.

Should range literals create a list of the passed-in type? It
might be desirable in the cases of other builtin types, such as
longs and strings:

>>> [ 1L : 2L<<64 : 2<<32L ]
>>> ["a":"z":"b"]
>>> ["a":"z":2]

However, this might be too much "magic" to be obvious. It might
also present problems with user-defined classes: even if the
base class can be found and a new instance created, the instance
may require additional arguments to __init__, causing the
creation to fail.

The PyList_FromRange() and PyList_GetLenOfRange() functions need
to be classified: are they part of the API, or should they be
made private functions?

After careful consideration, and a period of meditation, this
proposal has been rejected. The open issues, as well as some
confusion between ranges and slice syntax, raised enough questions
for Guido not to accept it for Python 2.0, and later to reject the
proposal altogether. The new syntax and its intentions were deemed
not obvious enough.

[ TBD: Guido, amend/confirm this, please. Preferably both; this
is a PEP, it should contain all the reasons for rejection
and/or reconsideration, for future reference. ]